1. Field of the Invention
This invention relates to technology for the treatment of toxic organic pollutants, also known as xenobiotics. More particularly, the present invention relates to a process for biodegradation of a xenobiotic using a two-phase partitioning bioreactor.
2. Description of the Prior Art
Xenobiotics are toxic organic compounds which are manufactured by major multinational chemical producers and are used extensively by downstream producers of plastics and other chemical products. They include compounds such as benzene, toluene, and styrene, as well as halogenated organic compounds such as pentachlorophenol and PCBs. These pollutants are often present in process waste streams in fairly low concentrations, or may be present in larger quantities as spills, or in the soil and water associated with abandoned industrial sites in North America and former Eastern Bloc countries. These compounds are generally highly toxic to life forms (including humans), are exceedingly difficult to dispose of, and are of major concern to industry (because of the cost and/or difficulty of treatment) and to regulatory agencies.
Xenobiotics, whether they occur in process waste streams or in spills, can be treated by physical, chemical and biological means. For example, air and water streams containing these toxic compounds can be contacted with activated carbon particles contained within adsorption columns, and thus these materials are physically removed. A significant drawback of this approach is that the xenobiotics adsorbed onto the carbon are not destroyed, only physically removed from the contaminated stream, and therefore some subsequent disposal method must be employed as a final means of destruction. Xenobiotics may also be removed by incineration (a chemical method), however this approach requires costly high temperature and/or pressure equipment and may also release undesirable combustion products to the atmosphere. Moreover, if the xenobiotic is present in the contaminated stream at low concentrations, very large volumes need to be incinerated, with associated high costs. Biological treatment of xenobiotics involves the addition of these materials to bioreactors (essentially stirred tanks containing aqueous suspensions of microorganisms) which operate at ambient conditions, and which can ultimately degrade these materials to harmless end products such as carbon dioxide and water. Although potentially the lowest cost approach to xenobiotics destruction, current biological treatment of toxic organics has a fundamental inefficiency.
In the simplest form of biological treatment xenobiotics are added to conventional biological wastewater treatment systems, with the hope that the microorganisms naturally present in such systems will degrade (typically by metabolization) and eliminate these compounds. However, even though xenobiotics can be "food" to microorganisms, they are toxic compounds and, if added in too high a concentration or too quickly, can kill all of the microorganisms present in a conventional wastewater treatment system. If added too slowly, the microorganisms present in a biotreatment system could starve, or could lose their ability to consume these compounds. The basic inefficiency, and problem, therefore, is how to deliver these compounds to a xenobiotic biotreatment systems in a controlled fashion, fast enough not to starve the microorganisms, but not so quickly so as to kill them.
In particular, phenol has been a troublesome pollutant in the environment throughout the last century. It is present in many industrial effluents and is currently removed by costly and relatively inefficient physical or chemical methods. Current methods of treatment often produce other toxic end products as well, requiring further processing steps (Kobayashi et al..sup.1).
Microorganisms that can degrade phenol were isolated as early as 1908 (Evans.sup.2) Current technology permits the use of these microorganisms in batch and continuous processes, using either suspended or immobilized cultures. The difficulty associated with many conventional batch reactors is that the initial substrate concentration must be lower than the value at which the organisms are inhibited, which, for most xenobiotics, results in a very low concentration of xenobiotic being degraded in a relatively long period of time. Increasing the initial substrate concentration in a batch reactor simply prolongs the process, by increasing the duration of the lag phase (Andrews.sup.3)--this results in an overall decrease in the efficiency of the process.
In a continuous culture, low dilution rates are necessary to avoid process instability or low conversion (Pawlowsky et al..sup.4). In addition to the low dilution rates required, most continuous cultures using an inhibitory substrate will have higher productivities and increased stability if the cells are immobilized. As such, the surface area within the fermentation vessel must be maximized to promote wall growth (Molin et al..sup.5). However, biofilm formation within a reactor leads to difficulties in operational control, and requires very high levels of aeration and agitation to deliver the required oxygen and nutrients to the immobilized cells.
Two-phase bioreactor systems have previously been used in extractive fermentation (Barton et al..sup.6 and Jones et al..sup.7). The organic phase in these fermentations is used to selectively remove the inhibitory end product from the aqueous phase as it is produced. This reactor scheme has numerous advantages, allowing for greater productivity within the system due to the absence of inhibition, and allowing for reduced water usage, as the substrate can be introduced at high concentrations without requiring prior dilution (Daugulis et al..sup.8). The overall concentration of the inhibitory substrate in the system is very high, but the concentration of substrate in the aqueous phase can be maintained well below inhibitory levels (Vermue et al..sup.9).
This configuration has been used to degrade styrene in a silicon oil/aqueous system (El Aalam et al..sup.10); however, the solvent used was not systematically selected in order to optimize the system.
Thus, although there have been advances made in the art, there is room for improvement. Specifically, it would be desirable to have a process which has improved efficiency and can be selectively operated in a batch, semi-batch (also known as "fed batch" or "sequential batch") or continuous mode to biodegrade a xenobiotic.